The Libertarian Party was founded 40 years ago to provide a conservative alternative to the existing two party system.
In 1972, the party first presidential ticket won 2 electoral votes thanks to faithless Electors in Virginia and Arizona. From then on, the Libertarians took off.
In 1980, the voters had enough with the malaise and corruption of the two party system especially the Rockefeller Presidency. Americans elected the first Libertarian President and the Libertarians won control of the House of Representatives and became the official opposition in the Senate.
Is there any way that the Libertarians could have remained a fringe unelectable party? My guess is that Ronald Reagan is not assassinated in 1975. A lot of his hardcore supporters migrated to the Libertarians especially after Barry Goldwater switched his support in 1979.